A typical switch mode power supply (SMPS) includes a switching transistor coupled to a primary winding of power transferring transformer for periodically applying an input supply voltage to the primary winding. It is known to operate the SMPS in a run mode of operation and in a standby mode of operation. During a run mode operation, pulses of current are developed in a secondary winding of the transformer at a high frequency and are rectified for periodically replenishing a charge in a filter or smoothing capacitor. An output supply voltage, developed in the capacitor, is connected to energize a load.
In the run mode of operation, the SMPS operates in a continuous mode. In the standby mode of operation, it may be desirable to operate the SMPS in a burst mode for reducing power dissipation. In a given cycle of the burst mode, the high frequency current pulses a are developed in the transformer windings. The current pulses are followed by a relatively long interval, referred to herein as dead time interval, of several milliseconds, in which no current pulse is produced. An advantageous type of a SMPS is a zero voltage switching SMPS.
In carrying out an inventive feature, in a zero voltage switching SMPS, switching on the transistor occurs when the voltage between the main current conducting terminals of the transistor is zero for minimizing switching losses. It may be desirable to operate the zero voltage switching SMPS in the burst mode, during standby.
In a zero voltage switching SMPS, embodying an inventive feature, the standby mode is initiated by disconnecting a run mode load from the filter capacitor via a switch. Thereby, the run mode load ceases consuming load current. Because the run mode load circuit is de-energized, a feedback loop of the SMPS causes the transistor to conduct in a substantially shorter duty cycle than in the run mode. The short duty cycle in successive switching cycles of the transistor causes the zero voltage switching SMPS to operate in a standby, burst mode.